PPU 960 Physics Note [Sem 1 :Chapter 4 - Work, Energy and Power]

00

STPM

2012

About Author:

Facebook: www.facebook.com/groups/josh.lrt Email: [email protected] [Mr. Josh]

Contact No: +6018-397 6808 [Mr. Josh]

Chapter 4 – Work, Energy and Power By : Josh, LRT

2012 © LRT Documents Copyrighted. All rights reserved. Page 1 of 7

Chapter 4 – Work, Energy and Power

4.1 Work

Work is defined as force applied along a distance.

The unit of work is or or .

Work done by a force is equal to the product of the force and the displacement in the direction

of the force.

Work done by constant force,

Where, is the angle between F and s.

Work done by variable forces

** Notes:

1. A graph of Force agains Displacement, the area is the work done.

2. It must be consider the only distance given depends on the question.

3. Energy is transferred

4. Movement has to occur and must be in same direction

5. The angle are in the range of

Work Done

𝑠

𝑊 𝐹 𝑑𝑥𝑠

0

To find the work done varies from 𝒙 𝟎 to

𝒙 𝒔 is:

Shaded area under the (𝑭 𝑎𝑔𝑎𝑖𝑛𝑡𝑠 𝒙) graph.

𝜃

𝐹

𝐹

𝐹

𝑠



Condition where Work Done on an object is zero

i. While pushing a wall

ii. Force exerted perpendicular to displacement

Mathematically Proven for WORK

When

1. Force and displacement are rightward.

2. Force left, displacement right

3. Force up, displacement right

𝜃 𝐹

𝑠

𝜃 𝐹

𝑠

𝜃 9 𝐹

𝑠

𝑊 𝑭 𝒔 𝐜𝐨𝐬 𝜽

𝑭 𝟎 𝐜𝐨𝐬 𝟎

𝐽

As you are pushing the wall. Since, there is

no displacement made, so 𝒔 𝟎.

𝑊 𝑭 𝒔 𝐜𝐨𝐬 𝜽

𝑭 𝟏𝟎𝟎 𝐜𝐨𝐬 𝟗𝟎

𝐽

As waiter are carry the dishes. Since, there

the distance from a point to end is 𝒔 𝟏𝟎𝟎.



QUESTION:

The figure above shows a block X of mass m moving up a plane inclined at an angle to the

horizontal, whereas block Y of mass M is attached to block X with a non-elastic string over a pulley

and falls through a vertical height h. If the frictional force acting ott block X is F, then the heat

generated to overcome the friction is

A. B. C. D.

𝜃

𝑋

𝑌

𝐹

𝑆𝑚𝑜𝑜𝑡 𝑃𝑢𝑙𝑙𝑒𝑦



4.2 Energy

Spring,

o There is compression and extension.

SPRING COMPRESSED

SPRING EXTENDED

o Whether the spring is compressed or extended, the relationship of spring changes can be

expressed as below:

Mechanical Energy

Spring Potential Kinetic

Original Spring



According to Hooke's law,

Due to the force, F that applied on spring will not be constant! So, at first which is the early

stage – easy and later stage – harder.

A graph of F against x is plotted, the area under the graph is represent the work done by the spring.

The equation is

Potential Energy,

It is the energy due to its relative position or physical condition of a body.

Potential Energy

ELASTIC GRAVITATIONAL



Elastic Potential Energy → The energy stored in the spring when its extension is x.

0

0

[ ]

[ ]

Gravitational Potential Energy →

Kinetic Energy,

Kinetic energy of a body can be define as the amount of work it can do in coming to rest or the

energy which possesses solely because it is moving.

Let’s derive an expression by using the situation below,

Block M is stationary before any forces were applied. After a few seconds, one idiot pushes the

block and its move with a velocity v and travel for a distance s. Given that .

0

0

[ ]

0

0

GPE of an object is the energy it possesses by virtual of

its position in a gravitational field or can be defined as the

amount of work that was done on it to give it that energy.

𝑣0 𝑣

𝑀 𝑀 𝐹

𝑠

𝐹



Principle of Conservation of Energy

It stated that energy cannot be created or destroy. What happened in our daily life is the energy

transformed into another types of energy. The energy given or provided will be always constant

unless it had been transformed.

Conservation of Mechanical Energy

In a system, in which the only force acting are associated with potential energy, the sum of the

kinetic & potential energy is constant.

ADVICES:

For this chapter, the lesson notes will be short. What you guys need to do is:

1. Derive all the expression into a valid equation.

2. Solve problems on this chapter.

3. Do not memorize the situation but understand it.